Recent advances in projection television, interactive whiteboards, digital signage, liquid crystal displays and other display technologies have led to the need for the development of increasingly large display screens in both the transmission and reflection modes. High performance rear projection screens require balanced characteristics of resolution, gain, contrast and wide viewing angle. The screen must provide sufficient resolution to display sharp images for both video and data presentation and must, at the same time, cause diffusion of the light in such a way as to provide a wide viewing audience angle while still providing high brightness and high contrast, particularly for viewing in the presence of ambient light. A gray or black tint is generally incorporated in monolayer diffusion screens to provide this required contrast.
However, while it is necessary for these display technologies to provide a wide viewing angle in the horizontal direction, it is often desirable to have a much smaller viewing angle in the vertical direction, the requirements for this depending on the specific application. For example, in many display and projection screen applications, it is desirable to have a broad horizontal viewing angle because a viewer may be positioned to the side of the screen, while there is generally no need for the vertical viewing angle to be as broad because the viewer is generally positioned with the screen at or near eye level. Thus it can be desirable to have an anisotropic or elliptical display with a comparatively wide horizontal viewing angle but a comparatively narrow vertical viewing angle. This asymmetric or elliptical viewing field has the advantage that, since the total light energy transmitted through the screen is constant depending on the light engine, if the viewing field in the vertical direction is truncated or otherwise shortened, then more energy is available for increasing the horizontal viewing angle or for increasing the screen gain (brightness) or a combination of these.
Systems using lenticular lens designs where one-dimensional lens structures are molded onto plastic substrates are well known in the art and have become a major current projection television screen technology to provide such screens with an asymmetric viewing area. Light is focused by the cylindrical type of lens structure onto a second diffusive film or layer to produce asymmetric diffusion. However, such lenticular-based screens are limited in the size which can be manufactured, because of the practical limitations in size of the lenticular embossing roll, and in their resolution, which is limited by the lenticular pitch which can be manufactured. In addition, the lenticular-type lenses contain a series of grooves which can be expressed as a frequency, and this frequency can interfere with the pixel frequency in liquid crystal-based and other microdisplay-based projection displays and generate Moiré fringes. As a result, the currently available lenticular screens are limited for high definition image displays in which the pixel frequencies are higher.
Light scattering systems are known in the art which employ asymmetric scattering particles as a dispersed phase in a transparent matrix material to produce an asymmetric light scattering pattern. These systems fall into two basic types, one using incompatible polymers where the discontinuous phase particles are elongated by shear in processing to provide such asymmetric particles, and the other using preformed elongated organic or inorganic materials which are then oriented during processing to give the same effect.
The optical properties of the resultant asymmetric diffuser systems are determined by the refractive index difference between the matrix and the dispersed phase, and the size, location, shape, aspect ratio and orientation of the elongated structures of the dispersed phase. These factors can be varied to provide an optical element with predetermined optical properties. Thus, the largest scattering angles occur in directions substantially perpendicular to the major axes of the elongated diffuser structures, while conversely the smallest scattering angles occur in directions substantially parallel to the major axes of the elongated structures. Such properties are particularly useful for displays and projection screens.
With regard to asymmetric diffuser systems employing incompatible polymers, i.e., where the discontinuous phase scattering materials are elongated by shear during processing to provide elongated particles, U.S. Pat. Nos. 5,932,342 and 6,346,311 disclose such systems.
Similarly, Japanese Patent Application Kokai 314522/1992 describes an anisotropic light scattering material including a transparent matrix material and a dispersed phase of transparent diffuser particles for use for projection television screens. U.S. Pat. No. 6,582,783 discloses laminated films comprising an anisotropic light-scattering layer and a transparent resin layer laminated to at least one side of the light-scattering layer.
U.S. Pat. Nos. 6,727,313 and 6,819,468, and US Patent Application 20050036199 also disclose screens for managing light, including front and rear projection screens, including a substrate layer and a polymeric composition of an adhesive matrix material containing a dispersed phase of elongated structures disposed on the substrate.
Difficulties with the use of the above systems include the facts that they are either not applicable to the manufacture of rigid screens incorporating rigid diffuser layer elements, or the incompatible polymer systems are extremely difficult to control in a practical processing environment where it is necessary to make wide extruded or coextruded sheets with precisely controlled morphology, structures and thicknesses in order to provide the desired optical, physical and thermal properties to the resulting device. Moreover, it is difficult to provide stable colloidal systems so as to prevent any changes or reagglomeration of the diffuser particles or migration to the surface or an interface during sheet extrusion, thereby causing an undesirable deterioration or loss of the designed optical, physical and thermal properties. In addition, these systems do not show an optimal combination of optical properties such as brightness (gain), scattering angles in the horizontal and vertical directions, contrast and resolution.
Consequently, there is still a need for the development of improved inexpensive and efficient front- and rear-projection screens and displays with desirable combinations of the above light-scattering and transmission properties, in particular with controlled light scattering asymmetry, to provide such advantageous combinations of brightness, contrast, resolution and wide viewing angle in those directions which are important for the intended application and a narrower viewing angle in other directions where such wide viewing angle is unnecessary. This need is now addressed by the asymmetric viewing area optical display device of this present disclosure.